The present invention relates to a switching power supply circuit, and particularly to that to be applied to a power supply circuit of a portable OA (Office Automation) equipment such as a laptop personal computer.
Portable OA equipment such as a laptop computer driven with a battery is widely used, wherein a voltage regulator is used for generating necessary DC voltages, .+-.5V or .+-.12V, for example, being supplied from the battery having a normal voltage 8 to 16V. As to the voltage regulator, a switching power supply circuit is generally used in order to satisfy both the high conversion efficiency and the miniaturization required for the portable equipment.
FIG. 4 is a block diagram illustrating a prior example of the switching power supply circuit disclosed in a Japanese patent published with a Specification No. 20368/'95, comprising a drive circuit 103, a constant voltage circuit 1 for supplying a constant voltage Vb to the drive circuit 103, a control circuit 102 for generating a pulse signal used in the drive circuit 103 for determining duty-cycle of the switching operation, an nMOS (n-type Metal Oxide Semiconductor) transistor Q1 with its gate controlled by the drive circuit 103 for switching an input DC voltage VIN to be output as a switched voltage Va, and a LPF (Low-Pass Filter) having a diode D2, a choke inductor L1 and a capacitor C2 for obtaining an output voltage VO by smoothing the switched voltage Va.
For efficiently switching the input DC voltage VIN, gate of the nMOS transistor Q1 should be controlled with ON voltage sufficiently high to its source voltage (=switched voltage) Va. For the purpose, the drive circuit 103 is provided in the prior art.
The drive circuit 103 comprises transistors Q11, Q12, Q14 and Q15, a diode D1 for preventing backward current, and a booster capacitor C1 connected to the gate of the nMOS transistor Q1.
The constant voltage circuit 1 outputs a constant DC voltage Vb, which charges an electrode, connected to collector of the transistor Q11, of the capacitor C1 through the diode D1 to a charged voltage Vc, when potential of another electrode of the capacitor C1 is lower than the constant voltage Vb coupled to the ground level GND through gate capacitance of the nMOS transistor Q1 at status OFF.
When the transistor Q11 of the drive circuit 103 is controlled to be ON, the charged voltage Vc is impressed to the gate of the nMOS transistor Q1 and turns ON the nMOS transistor Q1, raising up the switched voltage Va. When the switched voltage is raised up, it pushes up the charged voltage Vc, that is, the gate voltage of the nMOS transistor Q1 through the gate capacitance.
Thus, the nMOS transistor Q1 is sufficiently driven by the charged voltage Vc thus pushed up.
However, the capacitor C1 is charged in intervals when the nMOS transistor Q1 is at status OFF. Hence, it can not be charged sufficiently when duty-cycle of the switching operation becomes high according to fall of the input DC voltage VIN because of discharge of the battery supplying the input DC voltage VIN, resulting in insufficient gate voltage for driving the nMOS transistor Q1, which limits available life of the battery before the duty-cycle approaches to 100%.
Furthermore, although higher switching frequency is more preferable for miniaturization of the switching regulator, the above phenomenon means that the minimum operational voltage of the input DC voltage VIN also becomes higher for securing necessary charge to drive the nMOS transistor Q1, when the switching frequency is made higher, making the available battery life still shorter.